The invention relates generally to wavelength division multiplexed transmission systems, and more particularly to a transmitter employed in wavelength division multiplexed transmission systems which increases spectral efficiency by reducing four-wave mixing.
Wavelength-division multiplexing is expected to be increasingly utilized in undersea and transcontinental terrestrial optical transmission systems due, in part, to the large bandwidth capacity such multiplexing provides. One way of increasing the total transmission capacity of such systems is to more efficiently use the available spectral bandwidth such as by decreasing the spacing between adjacent ones of the multiplexed channels. Unfortunately, wavelength division multiplexed transmission systems are susceptible to performance limitations due to polarization dependent effects such as cross-talk between the multiplexed channels. Cross-talk, which is primarily caused by the non-linear index of refraction of optical transmission fibers, increases as the channel spacing decreases. Four-wave mixing is one significant deleterious effect that produces cross-talk.
U.S. application Ser. No. 09/031,018, now U.S. Pat. No. 6,134,033 discloses an optical transmitter that generates a WDM signal having even-numbered channels in a state of polarization (SOP) orthogonal to the SOP of the odd-numbered channels. This arrangement advantageously limits the four-wave mixing products that can be generated in the transmitter and the optical transmission path to which it is typically coupled.
Wavelength division multiplexed systems must also employ dispersion management techniques. As the per channel data rates of such system increase, the interplay of dispersion and fiber nonlinearity needs to be more carefully managed. Typically the transmission line is designed to have an average dispersion value of zero. In the case of WDM systems with a non-zero dispersion slope, however, only one channel can be arranged to have an average dispersion of zero. The remaining channels will have some net nonzero dispersion due to the dispersion slope of the optical fibers forming the transmission line. One technique for overcoming this limitation at intermediate points along the transmission path (i.e., in the undersea plant in undersea transmission systems) in WDM systems is disclosed in U.S. application Ser. No. 08/759,493, now. U.S. Pat. No. 6,137,604. As discussed therein, it is useful to divide the usable optical bandwidth of the transmission system into sub-bands that individually undergo dispersion compensation before being re-combined. In comparison to other dispersion compensation techniques, more WDM data channels reside near a wavelength corresponding to the average zero dispersion wavelength. However, to implement in a straightforward manner this dispersion management technique at the transmitting terminal, in connection with a signal that has SOPs which are pairwise orthogonal, requires a dispersion compensator that maintains the SOPs of the optical signal.
Unfortunately, a transmitter that offers both dispersion compensation and a signal in which adjacent channels have orthogonal SOPs is difficult to provide because of the unavailability of a simple and inexpensive means for performing dispersion compensation in a polarization maintaining environment.
Accordingly, it would be desirable to provide a transmitter that generates a dispersion-compensated WDM optical signal having SOPs that are pairwise orthogonal without the need for a dispersion compensating element that does not change the SOP of the optical signal.
In accordance with the present invention, a method and apparatus is provided for transmitting an optical signal having a total number of channels that are dividable into a prescribed number of wavebands. The method includes the step of generating a first series of optical signals corresponding to each of the wavebands. The first series of optical signals includes a plurality of optical channels, which are sequentially numbered from 1 to N from lowest to highest wavelength. Within each waveband, a state-of-polarization of predetermined odd-numbered channels is oriented to be substantially orthogonal to a state of polarization of predetermined even-numbered channels by directing the predetermined odd-numbered channels and the predetermined even-numbered channels through orthogonally polarizing inputs of a polarization coupler. The odd-numbered channels and the even-numbered channels within each waveband may be directed through first and second wavelength combiners, respectively, prior to orienting their states of polarization. The orthogonal relationship between the states of polarization of odd and even-numbered channels within each waveband advantageously limits the four-wave mixing products that can be generated in the optical transmitter and the optical transmission path to which it is typically coupled.